Microwave isolator



June 28, 1960 a. J. DUNCAN MICROWAVE IsoLA'roR 2 Sheets-Sheet 1 FiledJuly 10, 1956 M M R C Y o N E T u N wpd@ vd. N .n YJA B I Bm v zom Qmflfllllln lll! mm D.

June 28, 1960 Filed July 10, 1956 B. J. DUNCAN MICROWAVE ISOLATOR 2Sheets-Sheet 2 INVENTOR BoB/ay -J DUNCAN ATTORNEY MICROWAVE ISOLATORBobby J. Duncan, Port Washington, N.Y., assignor to Sperry RandCorporation, a corporation of Delaware Filed July 10, 195'6, Ser. No.596,907

3 Claims. (Cl. S33- 24) This invention relates to microwave transducersand more particularly to non-reciprocal microwave transducers, such asisolators.

A non-reciprocal transducer is a device having yan energy transferfunction dependent on the direction of energy passage therethrough. Anisolator -is a non-reciprocal transducer which freely transfers energyin one direction, but prohibits passage of energy in the reversedirection. Isolators are employed, for example, in micro wavetransmission systems to prevent transmitting devices, such as ldystronsand magnetrons, from receiving waves reflected from loads,'such asantennas.

A load connected to a transmission means, such as a waveguide, does notpresent a perfect match to incident electromagnetic` waves and,consequently, a portion of these waves will be reflected at thejuncture.' l The combination of an incident and a reflectedelectromagnetic wave on a transmission means creates standing waves. Theeffect of such standing waves is to present to the transmitting devicean impedance which is not the characteristic impedance of thetransmission means. Any change in the load, such as occurs in a scanningradar, will be reflected toV the transmitting device as a varyingimpedance. This varying impedance seen by the transmitting device willnot only result in a vari` able amount of power being deliveredto theload, but may also result in pulling of the transmitting device; thatis, its frequency will change. It is the function of an isolator toprevent the reected wave from reaching the transmitting device, but tofreely permit passage of the incident wave. This may be accomplished byproviding a non-reciprocal attenuator;` that is, by attenuating thereflected wave and allowing the incident wave to pass unattenuated. inthis Way the transmitting de -V vice sees only the characteristicimpedance of the transmission means regardless of changes `in the load,y

One way in which isolation has been accomplished is describedin U.S.patent application Serial No. 551,872, tiled December 8, 1955, by B. I.Duncan, assigned to the same assignee as the instant invention.V A'ferrite member disposed within a section of waveguide employs a hollowstructure, the hollow portion being lled with a lossy material. A sourceof magnetomo-tive force produces la magnetic field for immersing theferrite memr` ber. The value of magnetic ield employed is that valuewhich will magnetically bias the ferrite to zero effective permeabilityfor the `incident electromagnetic Wave. The incident wavewill besubstantially excluded from the ferrite member andwill propagate past itsubstan-A tially unattenu-ated. However, as the ferrite member presentsa finite permeability to the reflected Wave, this wave will passtthroughthe ferrite memberand be at'- tenua-ted therein. This device performsvervvvell as an isolator at, relatively vlow and medium microwavepowers.,Its ability to handle high' microwave powers is somewhat`limited''because the Yreiiecte'd wave is entirely slissip'atedA withinthe ferrite member.

2,943,274 Patented .lune 28, 1960 It is therefore the principal objectof this invention to provide an improved isolator capable of handlinghigh 657"urated' ferromagnetic material is not a scalar quantity,

values of microwave power.

microwave isolator in which the heat generated by the dissipated wave iseasily removed.

It is a further object of this invention to provide a ferrite isolatorin which a coolant may be circulated about the ferrite member.

It is a further object of this invention V,to provide an isolator inwhich the reflected wave is dissipated in a circulating coolant.

It is a further object of this invention to provide a ferrite isolatorin which a circulating coolant is emv ployed both for removing heat fromthe ferrite member. and for dissipating the reflected wave.

In accordance with the present invention, a cylindrical ferrite member'is magnetically biased to zero effective permeability for positivelyrotating circularly polarized waves. The ferrite member is coaxiallydisposed Within a circular waveguide section, and surrounded byavconductiveshell. A lossy coolant is circulated between the shell andthe inner surface yof the circular waveguide section. The'incident wave,rotating in the negative sense, passes through the ferrite member in adielectric waveguide mode.` The reflected wave, rotating in the positivesensei,l is rejected by the ferrite member, and passes around theferrite member and conductive shell and through the coolant where it'is'dissipated By thus dissipating theireected Wave ina circulatingcoolant, which also removes heat from Vthe ferrite meml.

b er, this isolator isl capable of dissipating high micro" wave powers.i' "11: The present invention will be described with reference to thefollowing drawings, wherein:

Fig.' 1 is a graph of the real part of the effective -permeability forferrites as `a function of applied magnetic: field;

Fig. 2 is an elevational view, partly in cross-section, 0f the isolatorof this invention; Fig. V3 is a perspective'view, partly inVcross-section of the device ofFig. 2; u

Fig. 4 is a series of drawings of the electromagnetic fieldvcontiguration at variousr points in the isolator of Fig.2. j Y `1Ferrites can `be described as polycrystallinematerials of spinelstructure which are formed Vat 'high temperature by the solid-phasereactions of iron oxide and one or more 4divalent metallic oxides.v Byvarying the ingredients and the processing techniques, Awide ranges inthe general properties of ferrites canbe obtained. Ferrites in theirsimplest form correspond to the general chemicaly formula XOFegOS, whereX represents thedi'- valent metal. e eral formula fall into two `mainclassesi those Whichvare ferromagnetic and those which are not.`Whethera ferrite falls into one or theV other of these classes dependson the divalent metallic oxide used. For example, those ferrites inwhich X `is magnesium, copper, manganese; lithium, nickel, lead, iron,calcium o r cobalt arev ferromagnetic. The ferromagnetic ferrites `areceramic-like materials Vcharacterized by low conductivity, low losses,-and high permittivity. l Y l' It is'vvell known that the RlF.permeability of a sati but instead the alternatingflux density inthemedium' is relatedto the'alternatin'g eld by a tensor permeabil-fV 1O ,Yity. The tensorY components ofthehperme'ability are;v complexquantities. This uniqueVV tensor permeability'i's' e v the property offerrites that makes themnseful for norfreciprocal devices.

Ferrites representedy by the above gen- Oppositely rotating circularlypolarized magnetic field components of electromagnetic waves encounterdifferent propagation constants in a ferrite due to its tensorpermeability. The component permeabilities, which are complexquantities, encountered by the vtwo circularly polarized componentsdepend on the material, the frequency of the wave, and the strength ofthe applied maghetic field. The rear part of the permeability presentedto the circularly polarized magnetic field component by a ferromagneticferrite which is magnetized perpendicularly to the plane of rotation ofthe magnetic field component is shown in yFig. 1 as a function of thebiasing magnetic field intensity HB. This curve will be similar fordifferent -ferrites and different frequencies and will differ only inmagnitude and the positions of critical points.

The propagation constant o'f an `electron'iaguetic wave in a medium isproportional to the factor (ne) V, where n is the permeability and e thepermittvity of the medium. If the permeability encountered by a wave iszero, the propagation constant does not exist and the wave will notpropagate in the medium. In such a ca'se a wave will be substantiallyexcluded from the medium and will not penetrate into the medium. Thus,the medium will act similarly Yto a conductor;

Referring once more to Fig. l, it may be noted that for a `givenmaterial two values of biasing magnetic field exist for which the realpart of the permeability is zero for the positively rotating magneticfield component of the wave. The lesser of thse two magnetic fieldvalues is designated H and is the preferred operating point. Using thelesser value, H0, of the field, adjustment of the biasing field is lesscritical and the source of field may be smaller. Furthermore, it hasbeen shown that a ferrite so biased will exhibit zero permeability for abroad range of frequencies. If the imaginary part of the permeability isvery small when the real `part is zero, the total effective permeabilityis practically zero and the positively rotating waves will not propagatein the medium. On the other hand, with this value of biasing magneticfield, the permeability is finite for the negatively rotating magneticfield component, and this wave compo'- nent will propagate through theferrite. Many ferrites eX- hibit both a low imaginary part and a zeroreal part of the permeability for waves of one sense of rotation atcertain frequencies. In particular, magnesium-manganese ferrites,nickel-zinc ferrites, and many nickel ferrites display thischaracteristic. The commercially available Ferran-lic R-l product of theGeneral Ceramics Corporation has thisftype of permeability.

The preferred embodiment of the isolator of this nvention, shown inFigs. 2 and 3, Aincludes a circular waveguide section having coaxiallydisposed therein a cylindrical ferrite rod v11. Ferrite rod 11 istapered at each end to effect wave matching properties. Surroundingferrite 'rod 11 and coaxially disposed therewith is a conductive shell12 composed of a material such as brass or copper. The cylindricalsurface of ferrite rod 11 is somewhat longer than shell 12, in orderthat the rod may readily interact with electromagnetic waves travelingthrough the waveguide section in each direction. A tapered conductivemember 14 is disposed between Vshell 12 and the inner surface ofwaveguide section 10 near one end of `said shell and serves to helpsupport ferrite rod 11 and shell 12 within waveguide section 10. A'tapered dielectric member 15 is disposed adjacent conductive member 14.A tapered dielectric member 17 is disposed between shell 12 and theinner surface of waveguide section 10 near the other end of said shelland serves to help support ferrite rod 11 and shell A12. p Bothconductive Ymember 14 and dielectric member 17 are sealed to shell 12andthe inner surfaces 'of waveguide section 10 in a fluid-tight manner. Aconical dissipative member 18 is disposed adjacent dielectric member 17and between shell -12 and the -inner surface of waveguide section 10.Conduits 20 and 21 in the wall of waveguide section 10 serve to direct afluid coolant through the chamber defined by shell 12, the inner surfaceof waveguide section 10, conductive member 14, and dissipative member18. A source of biasing magnetic field, such as a solenoid 23, surroundswaveguide section 10 and supplies an axially directed biasing magneticfield HB for ferrite rod 11.

A pair of circular polarizes 25 and 26, disposed within waveguidesection 1t) opposite each end of ferrite rod 11, serve to convertlinearly polarized waves to circularly polarized waves and vice versa.Such devices may take the form of conductive or dielectric quarterWavelength plates. `In the instant invention quarter wavelengthdielectirc plates are illustrated. A pair of circular-to-rectangularwaveguide transitions 28 and 29 connect circular waveguide section 10 toa pair of rectangular waveguide sections 3i) and 31. A pair ofconductive plates 34 and 35, oriented parallel to the broad walls ofrectangular waveguide sections 30 and 31, are disposed in respectivetransittions 28 and 29. The planes of circular polarizers 25 and 26 arerotated 45 from the planes of conductive plates 34 and 35.

To clarify the ensuing explanation of the operation of this device, thefollowing definitions are adopted:

Clockwise rotation-The rotation of a wave which appears to turn in aclockwise manner when viewed in the direction of propagation of thewave.

Counter-clockwise rotation- The rotation of a wave which appears to turnin a counter-clockwise manner when viewed in the direction ofpropagation of the wave. e

Positive rotation.-Rotation in the direction of the positive electriccurrent which creates a steady longitudinal magnetic eld.

Negative rotatz'on.-Rotation in the direction opposite the positiveelectric current which creates a steady longitudinal magnetic field.

The operation of this invention may be more readily understood byconsidering the drawings of the electromagnetic field configurations ofFig. 4 in conjunction with the structure of Figs. 2 and 3. All of thedrawings of Fig. 4 are illustrated looking toward the right in Fig. 2.The solid lines represent the electric field and the dotted linesrepresent the magnetic field. A transmitting device 37 launches alinearly polarized TE, wave, the dominant mode for rectangularwaveguide, into rectangular waveguide section 30. The fieldconfiguration in waveguide section 30 is shown in Fig. 4a, which is across-section of Fig. 2 at section A--A. This wave propagates to theright in the gure and is coupled through transition 2 8 into circularwaveguide section 10, where the fieldV configuration becomes that of thelinearly polarized TEu mode, the dominant mode for circular waveguide,shown in Fig. 4b, at section B--B. Plate 34 insures that the electricfield polarization in circular waveguide section 10 will beperpendicular to the extended broad walls of the rectangular waveguidesections. The wave of Fig. 4b continues toward the right and throughcircular polarizer 25, where it is converted to a clockwise rotatingcircularly polarized TE wave. The magnetic field component of thisrotating wave is shown in Fig. 4c at section CC.

The clockwise rotating wave is rotating in the negative sense withrespect to the biasing magnetic field HB applied to ferrite rod 11.Consequently, to this wave ferrite rod 11 acts as a dielectric member.Conductive member 14 and dielectric member 15 serve to guide this waveinto ferrite rod 11. Ferrite rod 11, when surrounded -by conductiveshell 12, acts as a dielectric-filled waveguide for the wave propagatingto the right. As the cutoff diameter for a dielectric-filled waveguideis very much smaller than that foran air-filled waveguide, the wave willpropagate toward the right inside shell 1,2.

One component of the circularly polarized eld for this wave is shown inFig. 4d, at section D-D. For best where ,ur is the relative permeabilityand e, is the relative permittivity of the dielectric-filled waveguide.Owing to the presence of conductive memberv 14 and shell 12 no portionof the wave can propagate to the right external to shell 12. Y'

The wave leaves ferrite rod 11 and reenters waveguide section throughdielectric Amember 17. If ferrite rod 11 is a low loss type this wavewill be substantially unattenuated. This wave is shown in Fig. 4e, atsection E-E. The clockwise rotating wave enters circular polarizer 26where it is reconverted to a linearly polarized wave, as shown in Fig.4f, at section F-F. The wave continues to the right, traveling throughtransi-v tion 29 to waveguide section 31 (Fig. 4g), where it isdelivered to a l-oad 38.

Load 38 `does not present a perfect match to waveguide section 31 andconsequently waves traveling to the right in waveguide section 31 willbe partially re flected. The resulting reected waves propagate to theleft through waveguide section 31 and transition 29. This wave passes tothe left through circular polarizer 26, where it is converted to aclockwise rotating circularly polarized wave, and continues to the leftin waveguide section 10. As the wave is traveling in the oppositedirection from that shown in Fig. 4e it will rotate in the oppositedirection from the wave shown in Fig. 4e. Consequently the wave will berotating in the positive sense with respect to the biasing magnetic eld.The value of the biasing magnetic field is that required for the ferritemember to exhibit zero effective permeability for a positively rotatingwave. Therefore, Ithe positive rotating reflected wave cannot propagatein the zero permeability medium and will be substantially excludedtherefrom. The wave will be unable to penetrate rod 11, but instead willpass around it and enter the region lbetween conductive shell 12 andwaveguide section 10 after traveling through dielectric member 17 anddissipative member 18. Between shell 12 and waveguide section 10 eachcomponent of the circularly polarized wave will propagate in the coaxialline TEM mode, as shown in Fig. 4h, at section D-D. if a dissipativematerial fills the space between shell 12 and waveguide section 10, thewave will be rapidly attenuated. If the wave is not completelydissipated in its travel through this coaxial space, it will bereflected from conductive member 14 and be further attenuated las ittravels back to the right. For best results, lossy iiuid should becirculated through thiscoaxial space by means of conduits 20 and 21.This iiuid will serve not only to dissipate the reflected wave but willalso act as a coolant to carry away heat which may be produced due toany losses in ferrite rod 11. Ordinary tap water is usually yfound to bea sufficiently dissipative coolant for this purpose.

Dielectric member 15 serves to match the dielectric waveguide comprisingshell 12 and ferrite rod 11 to Waveguide `section 10 for negativelyrotating waves traveling to the right. Shell 12 not only serves as theconductive member of a dielectricilled waveguide, but prevents anyportion of the incident wave from traveling through the lossy coolant.Dielectric member 17 serves to match waveguide section 10 to thedielectric-filled waveguide for negatively rotating waves traveling tothe right. Dissipative member 18 in cooperation with dielectric member17 serves to match the effective coaxial line section, comprising shell12, waveguide section 10, and its lossy coolant interior, to waveguidesection 1t) Ifor positivelyv rotating waves traveling to the left.

Thus, extremely high powers of reflected waves ma v' be handled by thisdevice without causing the ferrite rod v to rise to a temperature whichmay either destroy it or seriously hamper its operation.`

While theinvention has been described in its preferred embodiment, it isto be understood that the words which have been -used are words ofdescription rather than of limitation and that changes withinthe'purview of the appended claims may be made without departing fromthe true scope and spirit of the invention in its broader aspects. Whatis claimed is: f 1. An isolator comprising a section of circular waveguide adapted to propagate electromagnetic Vwaves at a given frequency,means coupled to one end of said waveguide 'for launching thereinpositively rotating circularly polarized waves at said frequency andmeans coupled to the opposite end of said waveguide section forlaunching therein negatively rotating circularly polarized waves at saidfrequency, a longitudinally magnetized ferrite element disposed parallelto the llongitudinal axis of said waveguide section and Ifilling only aportion of the transverse cross-section of said waveguide section, saidelement presenting zero effective permeability only to said positivelyrotating circularly polarized waves and being permeable andsubstantially lossless to said negatively rotating circularly polarizedwaves, a conductive shell surrounding said element, ka materialdissipative to electromagnetic waves disposed between said shell and theinner surface of said waveguide section, Vthe diameter of saidferriteelement and its permittivity and its permeabilityV to said negativelyrotating circularly polarized Waves being proportioned to providedielectric waveguide means for said negatively rotating circularly.polarized waves, whereby said negatively rotating circularly polarizedwaves propagate through said element substantially unattenuated and saidpositively rotating circularly polarized waves propagate substantiallyonly between said conductive shell Yand the inner surface of saidwaveguide and are attenuated -by said dissipative material.

2. An isolator comprising a section of circular waveguide, a cylindricalferrite rod coaxially disposed within said waveguide section and llingonly a portion of the transverse cross-section of said waveguidesection, said ferrite rod being substantially lossless toelectromagnetic waves at a given frequency and being magnetized in adirection parallel tothe longitudinal axis lof said waveguide section tozero eifective permeability for positively j rotating circularlypolarized waves, means for launching mode, and a material los'sy toelectromagnetic waves disposed between `said shell and the inner surfaceof said waveguide section, whereby said negatively rotating circularlypolarized waves propagate through said ferrite rod substantiallyunattenuated and said positively rotating circularly polarized wavespropagate only between said cylindrical shell and the inner surface ofsaid waveguide and are attenuated by said lossy material.

3. An isolator comprising a section of circular wave"V guide adapted `topropagate electromagnetic waves at a given frequency, means coupled toone end of said waveguide for launching therein positively rotatingcircularly polarized waves at said frequency and meanscoupled to theopposite end of said waveguide for launching therein negatively rotatingcircularly polarized waves at said frequency, a longitudinallymagnetized ferrite member` disposed parallel to the longitudinal axis ofsaid waveguide section and lling only a portion of the transversecross-section of said waveguide section, said memberrpresenting zeroeiective permeability only to said positively rotating circularlypolarized waves and being permeable and substantially lossless to saidnegatively rotating circularly polarized waves, a conductive shellsurrounding said member, the diameter of said member and itspermeability and its permittivity to said negatively rotating circularlypolarized waves being proportioned to provide dielectric waveguide meansfor said negatively rotating circularly polarized waves, a dielectricmember disposed between said shell and the inner surface of saidcircular waveguide section near said opposite end of said waveguidesection, a conductive member disposed between said shell and the innersurface of the circular waveguide near said one end of the waveguidesection, said dielec- 0 tric member and said conductive member beingsecured to the shell and the waveguide in a duid-tight manner whereby ailuid-tight chamber is formed, and means for circulating through saidchamber a iluid coolant which is dissipative to electromagnetic Waves atsaid given frequency.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Melchor et al.: Journal of Applied Physics, vol. 27, No. 1, Ian. 1956, pages 72-77. Copy in Scientific Library.

Measurement of the Microwave Properties of Ferrites at High PowerLevels, United States Department of Commerce, Oice of TechnicalServices, PB 1118 26, received in Patent Oice lune 26, 1956.

